In general, a lithographic printing plate consists of an oleophilic image area which is receptive to ink and a hydrophilic non-image area which is receptive to fountain solution in the process of printing. Lithographic printing is a method of printing which utilizes the nature of water and oily ink repelling each other, wherein printing is carried out by employing the oleophilic image area as an ink-receiving area and the hydrophilic non-image area as a fountain solution-receiving area (a non-ink-receiving area) in a lithographic printing plate and causing a discrepancy in the ink depositability at the surface of the lithographic printing plate, so as to result in ink depositing on the image area only and subsequently transferring the ink onto a printing substrate such as paper.
In constructing such a lithographic printing plate, a lithographic printing plate precursor having an oleophilic photosensitive resin layer (image-recording layer) formed on a hydrophilic support (a PS plate) has been widely used hitherto. A lithographic printing plate is usually prepared by exposing a lithographic printing plate precursor to light through a printing master such as a lith film and then removing the image-recording layer in the non-image area by dissolution with an alkaline developing solution or an organic solvent, while leaving an image-recording layer in the image area, thereby exposing the surface of the hydrophilic support.
The existing platemaking process for lithographic printing plate precursor necessitates a step of removing the non-image area after light exposure by dissolution with a developing solution which is compatible with the image-recording layer, or the like. However, elimination or simplification of such additional wet treatment is currently listed as a problem to be solved. Particularly, disposal of the waste solutions discharged from the wet treatment has recently attracted much industrial attention in view of the consideration for global environment. Thus, there is an increasing demand for a solution to the foregoing problem.
In this regard, there has been proposed, as a non-treatment (non-development) type not requiring wet treatment, a lithographic printing plate precursor that comprises a image recording layer in which the affinity to the fountain solution or the ink changes in accordance with exposure to light at the surface of the layer, thereby enabling printing without the removal of the image recording layer.
Further, as one of simple and convenient platemaking methods, there has been proposed a method so-called as on-press development method, in which an image-recording layer capable of the removal of the non-image area in a lithographic printing plate precursor in the course of the common printing process, is used to remove the non-image area on the printing press after exposure to light, thereby resulting in a lithographic printing plate.
Specific examples of the on-press development method may include a method of using a lithographic printing plate precursor having an image-recording layer that can be dissolved or dispersed in a fountain solution, an ink solvent or an emulsion of a fountain solution and ink; a method of mechanically removing an image-recording layer by means of contact with an impression cylinder or a blanket cylinder of a printing press; or a method of attenuating the cohesion of an image-recording layer or the adhesion between an image recording layer and a support by penetration of a fountain solution, an ink solvent or the like and then mechanically removing the image-recording layer by means of contact with an impression cylinder or a blanket cylinder.
In addition, unless specified otherwise, the term “development treatment process” as used in the invention means a step of removing an area unexposed to infrared laser light in the image-recording layer of a lithographic printing plate precursor by contacting the image-recording layer with a liquid (usually an alkaline developing solution) by using an apparatus other than a printing press (usually an automatic developing machine), in order to expose the surface of the hydrophilic support. On the other hand, the term “on-press development” means a method and its process of removing an area unexposed to infrared laser light in the image-recording layer of a lithographic printing plate precursor by contacting the image-recording layer with a liquid (usually a printing ink and/or a fountain solution) by using a printing press, in order to expose the surface of the hydrophilic support.
However, in the case of using the image-recording layer in the related-art image-recording method utilizing the ultraviolet ray or visible light, since the image-recording layer is not fixed even after exposure to light, it has been required to employ a troublesome method of, for example, storing the lithographic printing plate precursor after light exposure in a completely light-shielded state or under constant temperature conditions until the point of time to mount the lithographic printing plate precursor on the printing press.
Meanwhile, with the recent spread of digitalization technology of electronically processing, storing and outputting image information by computer, various new image outputting systems coping with such digitalization technology have been put to practical use. In this connection, attention has been given on the computer-to-plate technology of directly preparing a lithographic printing plate without the use of a lith film, by loading digitalized image information on a highly converging radiation ray such as laser light and scan-exposing a lithographic printing plate precursor with the same light. Accordingly, it is one of important technical subjects to obtain a lithographic printing plate precursor that is adaptable to such technology as described above.
As discussed above, a demand for simplification, introduction of dry processes, and elimination of treatment with respect to the platemaking operation is significantly increasing in recent years in the aspects of consideration on the global environment as well as adaptation to digitalization.
Recently, high output lasers such as semiconductor lasers and YAG lasers emitting infrared rays of 760 nm to 1200 nm in wavelength have become available inexpensively. Thus, it is highly expected that these high output lasers are utilized as an image recording means in a method of manufacturing a lithographic printing plate by scan-exposure, which can be easily incorporated into the digitalization technology.
In the related-art platemaking method, a photosensitive lithographic printing plate precursor is exposed to an image pattern at a low to medium illumination intensity, and the property changes in the image pattern induced from a photochemical reaction in the image-recording layer is utilized in the implementation of image recording. On the contrary, in a method of using the above-mentioned high output lasers, an area to be exposed is radiated with a large quantity of light energy in an extremely short period of time to convert the light energy efficiently into heat energy, this heat energy induces thermal changes such as chemical changes, phase changes, or morphological or structural changes in the image recording layer, and these changes are utilized in the implementation of image recording. Therefore, image information is input by means of light energy such as laser light, whereas image recording is achieved by means of a combination of light energy and reactions induced by heat energy. Typically, such a recording system making use of the heat generated by exposure to a high power density light is referred to as “heat mode recording,” and the conversion of light energy into heat energy is referred to as “photothermal conversion.”
A great advantage of the platemaking method of employing the heat mode recording is that the image-recording layer would not be sensitized under an ordinary level of illumination such as room light, and that fixation of an image recorded by exposure to a light of high intensity of illumination is not essential. That is, a lithographic printing plate precursor used in the heat mode recording is free from any fear of the precursor being sensitized by room light prior to the exposure proper, and fixation of image is not essential after light exposure. Therefore, for example, when a platemaking process is carried out in the on-press development mode, in which process an image recording layer which becomes insolubilized or solubilized by exposure using high output lasers is used to produce a lithographic printing plate having the light-exposed image-recording layer as the image pattern, it is possible to obtain a printing system which would not have an effect on the image even if it were exposed to the ambient room light after the exposure proper. Thus, it is expected to be possible with the use of the heat mode recording, to obtain a lithographic printing plate precursor which may be very appropriately used in the on-press development.
In this regard, Japanese Patent No. 2938397, for example, discloses a lithographic printing plate precursor having provided on a hydrophilic support, an image-formation layer in which hydrophobic thermoplastic polymer particles are dispersed in a hydrophilic binder. Japanese Patent No. 2938397 describes that it is possible to expose said lithographic printing plate precursor to an infrared laser and form an image by means of coalescence of the hydrophobic thermoplastic polymer particles by heat, and then to attach the plate precursor on the cylinder of the printing press and to develop on-press using a fountain solution and/or an ink.
However, it was discovered regarding the foregoing method of forming an image by coalescence of microparticles resulting from simple thermal fusion, that although the method exhibits good ability for the on-press development, it results in weak image intensity and insufficient resistance to printing.
For this reason, it has been proposed to improve the resistance to printing using a polymerization reaction. For instance, JP-A No. 2001-277740 describes a lithographic printing plate precursor having an image-recording layer (thermosensitive layer) containing microcapsules which comprise a polymeric compound, on a hydrophilic support. Furthermore, JP-A No. 2002-287334 describes a lithographic printing plate precursor having a support and an image-recording layer (photosensitive layer) containing an infrared absorbent, a radical polymerization initiator and a polymeric compound constructed thereon.
Also, a lithographic printing plate precursor using an iodonium salt and an infrared absorbent is known, but conventional lithographic printing plate precursors are in need of more improvement on the visibility of the drawn image after laser exposure as well as on the storage stability.